Back-illuminated type imaging device and fabrication method thereof

ABSTRACT

Light is illuminated from a back-surface side of a silicon substrate  4.  A back-illuminated type imaging device  100  reads out, from a front-surface side of the silicon substrate  4,  charges that are generated in the silicon substrate  4  in response to the illuminated light, so as to perform imaging. The back-illuminated type imaging device  100  includes pad portions  17  formed on the back surface of the semiconductor substrate  4,  and a plurality of pillars  9  that are formed in the semiconductor substrate  4,  are made of a conductive material and electrically connect wiring portions  12  formed on the front surface of the semiconductor substrate  4  and the pad portions  17  to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe Japanese Patent Application No. 2007-157460 filed on Jun. 14, 2007,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a back-illuminated type imaging device in whichlight is illuminated from a back-surface side of a semiconductorsubstrate and charges that are generated in the semiconductor substratein response to the light are read from a front-surface side of thesemiconductor substrate to perform imaging.

2. Description of the Related Art

The following back-illuminated type imaging device has been proposed.That is, light is illuminated from a back-surface side of asemiconductor substrate. The back-illuminated type imaging device storescharges that are generated in the semiconductor substrate in response tothe light, into charge storage areas formed on a front-surface side ofthe semiconductor substrate. The back-illuminated type imaging deviceoutputs signals corresponding to the charges stored in the chargestorage areas to the outside by a CCD or CMOS circuit formed on thefront-surface side of the semiconductor substrate so as to performimaging.

In order to absorb almost all of visible light, the semiconductorsubstrate (a photoelectric conversion area) the back-illuminated typeimaging device, the semiconductor substrate (a photoelectric conversionarea) is required to have a thickness of about 10 μm. For this reason,when the back-illuminated type imaging device is fabricated, at firststructures such as the charge storage areas and the CCD are first formedon the front surface of the thick semiconductor substrate. Thereafter, awiring layer is thereafter formed on the structures, and a supportingsubstrate is bonded thereto via an adhesive layer Then, thesemiconductor substrate is etched from the back-surface side until thethickness becomes 10 μm, for example. After the etching, structures suchas color filters and microlenses are formed on the back surface of thesemiconductor substrate so as to be aligned with the structures formedon the front-surface side of the semiconductor substrate.

In the back-illuminated type imaging device formed in such a manner, itis necessary to form a pad on the back-surface side or front-surfaceside of the semiconductor substrate so as to be connected to the wiringlayer formed on the front-surface side of the semiconductor substrate. Ageneral image sensor that is presently in wide use has such a structurethat a pad opening is provided on a light incidence side. In accordancewith this structure, test equipments for testing the functions of chipsin the wafer are installed in a fabrication line. For this reason, ifthe pad opening is formed on the front-surface side of the semiconductorsubstrate in the back-illuminated type imaging device, it becomesnecessary to drastically change or modify the test equipments, which mayincrease the fabrication cost.

Therefore, similarly to the general image sensor, various methods forproviding the pad opening on the light incidence side (the back-surfaceside) have been proposed for the back-illuminated type imaging device.One method of them is to form a through hole from the back-surface sideof the semiconductor substrate, thereby exposing from the back surfacethe pad connected to the wiring layer formed on the front-surface side(for example, see JP 2005-285814 A and JP 2006-19653 A). However, such amethod has the following problems. That is, if the through hole isformed before the color filter or the microlens are formed on theback-surface side, a material for the color filter or the microlens maybe left in the through hole and cannot be removed in subsequent steps.Or, since the substrate on which the material is to be deposited has alarge step formed thereon, the thickness of the deposited material isnot even, which may cause a fixed pattern noise that appears as obliquelines in an image. Moreover, even if the through hole is formed afterthe color filter or the microlens is formed, it is necessary to remove aphotoresist that defines the through hole without causing any harm tothe color filter or the microlens, which is very troublesome. Inparticular, as described above, in the semiconductor substrate having athickness of 10 μm, the above problems may become conspicuous, and itbecomes more difficult to perform such a removal work.

In light of the above, as disclosed in JP 2006-339566 A, the followingmethod may be conceived. That is, a trench is formed in thesemiconductor substrate so as to extend from the front surface of thesemiconductor substrate to reach the back surface thereof, the trench isfilled with a conductive material, and a pad is formed on the conductivematerial. It can be said that in principle, it is possible to fill thetrench corresponding to the pad with the conductive material. However,usually, the pad has a size of 100 μm×100 μm; therefore, in order toform a trench having substantially the same size and fill the trenchwith conductive material, if a CVD process is used, it is necessary toform a layer of the conductive material to a thickness equal to orlarger than 50 μm and then to remove the same thickness by ananisotropic etching process. Such a process step cannot be said to bepractical as a semiconductor fabrication process.

If the size of the trench is decreased, it may not cause any problem inview of a fabrication process. However, in such a case, there arises aproblem that the resistance of the conductive material filled in thetrench may increase, which may affect the device characteristics.

SUMMARY OF THE INVENTION

The invention has been made in view of such circumstances, and providesa back-illuminated type imaging device that can prevent the fabricationcost and the resistance between pads and wirings from increasing even ifpad openings is formed on a light incidence side.

According to an aspect of the invention, a back-illuminated type imagingdevice includes a semiconductor substrate, a pad portion, a wiringportion and a plurality of contact wiring portions. The semiconductorsubstrate includes a front surface and a back surface. Light isilluminated from the back-surface side of the semiconductor substrate.Charges that are generated in response to the light are read out fromthe front-surface side of the semiconductor substrate to performimaging. The pad portion is formed on the back surface of thesemiconductor substrate. The wiring portion is formed on the frontsurface of the semiconductor substrate. The plurality of contact wiringportions are formed in the semiconductor substrate and electricallyconnect the wiring portion formed on the front surface of thesemiconductor substrate and the pad portion.

In the back-illuminated type imaging device, each contact wiring portionmay be configured by a pillar made of a conductive material. The pillarextends from the front surface of the semiconductor substrate in adirection perpendicular to the front surface of the semiconductorsubstrate. The pillar reaches the back surface of the semiconductorsubstrate. The pad portion may be connected to one ends of the pillars.The wiring portion may be connected to the other ends of the pillars.

In the back-illuminated type imaging device, the plurality of pillarsmay be arranged two-dimensionally, when viewed in plan view.

In the back-illuminated type imaging device, portions of the pluralityof pillars at which the pillars have a maximum width may have 2 μm orless in length.

The back-illuminated type imaging device may further include analignment mark that is formed in the semiconductor substrate and is usedto align components on the front-surface side of the semiconductorsubstrate with components on the back-surface side. The alignment markmay be formed by a pattern of a plurality of pillars that are made of aconductive material and extend in a direction perpendicular to the frontsurface of the semiconductor substrate.

In the back-illuminated type imaging device, the plurality of pillars,which form the alignment mark, may be arranged two-dimensionally.

In the back-illuminated type imaging device, the pattern of theplurality of pillars, which form the alignment mark, may be symmetric.

In the back-illuminated type imaging device, portions of the pluralityof pillars at which the pillars have a maximum width may have 2 μm orless in length.

In the back-illuminated type imaging device, each of the pillars, whichform the alignment mark, may extend from the front surface of thesemiconductor substrate and may reache the back surface of thesemiconductor substrate.

The back-illuminated type imaging device may further include asupporting substrate bonded to the front surface of the semiconductorsubstrate via an inorganic adhesive layer.

In the back-illuminated type imaging device, the supporting substratemay be made of the same material as the semiconductor substrate.

The back-illuminated type imaging device may further include a film thatis made of the same material as the pad portion and is formed above theback surface of the semiconductor substrate. The film includes at leastone of (i) a light shielding film for shielding a part of pixel portionsformed in the semiconductor substrate from light and (ii) a lightshielding film for shielding a boundary between the pixel portions fromlight.

According to another aspect of the invention, a method for fabricating aback-illuminated type imaging device in which light is illuminated froma back-surface side of a semiconductor substrate, and charges that aregenerated in response to the light are read out from a front-surfaceside of the semiconductor substrate to perform imaging, includes:forming a plurality of first through holes in a plane area, on which apad portion to be formed on the back surface of the semiconductorsubstrate is to be formed, from the front-surface side of thesemiconductor substrate so that the through holes extends from the frontsurface of the semiconductor substrate and reaches the back surface;filling the plurality of first through holes with a conductive materialto form a plurality of first pillars made of the conductive material;forming a wiring portion on the front surface of the semiconductorsubstrate so that the wiring portion is connected to the plurality offirst pillars; and after the wiring portion is formed, forming the padportion on the back surface of the semiconductor substrate so that thepad portion is connected to the plurality of first pillars.

In the method for fabricating the back-illuminated type imaging device,in the forming of the first through holes, the plurality of firstthrough holes may be formed so as to be arranged two-dimensionally, whenviewed in plan view.

In the method for fabricating the back-illuminated type imaging device,in the forming of the first through holes, the plurality of firstthrough holes may be formed so that portions of the first through holesat which the through holes have a maximum width have 2 μm or less inlength.

In the method for fabricating the back-illuminated type imaging device,in the forming of the first through holes, a plurality of second throughholes may be formed in a plane area, in which a alignment mark that isused to align components on the front-surface side of the semiconductorsubstrate with components on the back-surface side are to be formed, sothat the second through holes extends from the front surface of thesemiconductor substrate and reaches the back surface. In the filling ofthe first through holes, the plurality of second through holes may befilled with the conductive material to form a plurality of secondpillars made of the conductive material. The alignment mark may beformed by the pattern of the plurality of second pillars.

In the method for fabricating the back-illuminated type imaging devicein the forming of the first through holes, the plurality of secondthrough holes may be formed so as to be arranged two-dimensionally, whenviewed in plan view.

In the method for fabricating the back-illuminated type imaging device,in the forming of the first through holes, a pattern of the plurality ofsecond through hole may be formed so as to be symmetric.

In the method for fabricating the back-illuminated type imaging device,in the forming of the first through holes, the plurality of secondthrough holes may be formed so that portions of the second through holesat which the through holes have a maximum width have 2 μm or less inlength.

The method for fabricating the back-illuminated type imaging device mayfurther include forming pixel portions in the semiconductor substrate,the pixel portions having charge storage areas for storing therein thecharges. The forming of the pad portion may include forming a film of aconductive material having a light shielding property, on the backsurface, and removing the conductive material at portions outside atleast one of a portion above a plane area in which the pad portion isformed, a portion above a part of the pixel portions, and a portionabove a boundary between the pixel portions.

The method for fabricating the back-illuminated type imaging device mayfurther include: after the pad portion is formed, forming color filtersabove the pixel portions, so as to correspond to the pixel portions; andafter the pad portion is formed and before the color filters are formed,heating the semiconductor substrate.

In the method for fabricating the back-illuminated type imaging device,the semiconductor substrate may be a first semiconductor substrate of aSOI substrate. The SOI substrate includes the first semiconductorsubstrate, a second semiconductor substrate, and an oxide filmsandwiched between the first and second semiconductor substrates. Themethod may further include: after the wiring portion is formed, bondinga supporting substrate on the front-surface side of the firstsemiconductor substrate via an inorganic adhesive film.

In the method for fabricating the back-illuminated type imaging device,the supporting substrate may be made of the same material as the firstsemiconductor substrate.

According to the above configurations and methods, it is possible toprovide a back-illuminated type imaging device capable of preventing thefabrication cost and the resistance between pads and wirings fromincreasing even if a pad opening is provided on a light incidence side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a back-illuminated type imagingdevice according to an embodiment of the invention, when viewed from aback-surface side.

FIG. 2 is a schematic sectional view taken along the line A-A of FIG. 1.

FIG. 3 is a schematic sectional view showing the back-illuminated typeimaging device 100 during fabrication process steps.

FIG. 4 is a schematic sectional view showing the back-illuminated typeimaging device 100 during the fabrication process steps.

FIG. 5 is a schematic sectional view showing the back-illuminated typeimaging device 100 during the fabrication process steps.

FIG. 6 is a schematic sectional view showing the back-illuminated typeimaging device 100 during the fabrication process steps.

FIG. 7 is a schematic sectional view showing the back-illuminated typeimaging device 100 during the fabrication process steps.

FIG. 8 is a schematic sectional view showing the back-illuminated typeimaging device 100 during the fabrication process steps.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

FIG. 1 is a schematic plan view of a back-illuminated type imagingdevice according to an embodiment of the invention, when viewed from alight incidence side (a back-surface side). FIG. 2 is a schematicsectional view taken along the line A-A of FIG. 1.

As shown in FIG. 1, the back-illuminated type imaging device 100includes an imaging area 30 for receiving light to perform imaging; amark forming area in which formed are a large number of alignment marksM that are used to align components (charge storage areas for storingcharges, a signal output portion for outputting signals corresponding tothe charges stored in the charge storage areas, and the like) on afront-surface side opposite to the back surface, in which the imagingarea 30 is formed, with components on the back-surface side; and a padforming area in which a large number of pad portions 17 that areconnected to wiring portions formed on the front-surface side are formedfor inputting and outputting signals from and to the outside.

As shown by the enlarged view in FIG. 1, each alignment mark M is formedof a pattern of nine pillars 8 which are made of a conductive material(e.g., doped polysilicon) and which are two-dimensionally arranged inthe vertical and horizontal directions. As shown in the figure, the ninepillars 8 are symmetric in the vertical and horizontal directions. Eachpillar is a circular column. Since the pattern of the nine pillars 8 hasthe symmetrical arrangement, it is possible to detect one alignment markM by using the nine pillars 8. As shown in FIG. 2, the pillars 8 areformed in an n-type silicon substrate 4, which is a semiconductorsubstrate, so as to extend from the back surface and reach the frontsurface.

As shown in FIG. 1 (enlarged view) and FIG. 2, the pad portions 17 areconnected to the plurality of pillars 9, which are made of theconductive material (for example, the doped polysilicon) and are formedin the silicon substrate 4 below the pad portions 17 so as to extendfrom the back surface and reach the front surface. The plurality ofpillars 9 are connected to wiring portions 12 formed on thefront-surface side of the silicon substrate 4. As shown in the enlargedview of FIG. 1, the pillars 9 are arranged two-dimensionally so as to besymmetrical in the vertical and horizontal directions. Each pillar 9 isa circular column. Since the pillars 9 only need to electrically connectthe pad portions 17 and the wiring portions 12 to each other, it is notnecessary that the pillars 9 are arranged symmetrically. Moreover, it isnot necessary that the pillars 9 are arranged two-dimensionally.

As shown in FIG. 2, when the back-illuminated type imaging device 100 isused, light is incident thereon from the back-surface side of thesilicon substrate 4. Specifically, light is focused by microlenses 21and filtered by color filters 20, and then the light is incident to thesilicon substrate 4. Charges that are generated in the silicon substrate4 are stored in charge storage areas 10 and signals corresponding to thestored charges are output to the outside by a MOS circuit including MOStransistors. In this specification, a positional relationship amongcomponents will be defined using the front and back surfaces of thesilicon substrate 4 as a reference. For example, when the front surfaceof the silicon substrate 4 is used as the reference, the light incidencedirection is defined as an upper direction of the front surface, whilewhen the back surface of the silicon substrate 4 is used as thereferences, the direction opposite to the light incidence direction willbe defined as an upper direction of the back surface.

The back-illuminated type imaging device 100 includes the siliconsubstrate 4, an insulating film 5 that is made, for example, Of SiO₂ andis formed on the front surface of the silicon substrate 4, an insulatingfilm 14 formed on the insulating film 5, and a supporting substrate 16formed on the insulating film 14 via an adhesive film 15.

The adhesive film 15 is formed of an organic material film made such asSiO₂ and is formed by a CVD process. The supporting substrate 16 ispreferably made of a material having a thermal expansion coefficientclose to that of the silicon substrate 4. This is because Such selectionof material can decrease damages to the silicon substrate 4, which maybe caused by a sintering process described later. However, if thesupporting substrate 16 is made of a material such as Pyrex which isincluded in such materials, white defects may occur due to theradioactive rays (especially, an a ray) emitted from radioactiveisotopes contained in the material. The white defects may occur evenafter the back-illuminated type imaging device 100 is assembled into acamera, and the defects are referred to as post-generated white defects.The post-generated white defects may cause a serious problem since theyare not subjected to an image correction process. For this reason, it ismore preferable that the supporting substrate 16 is made of a materialthat has a thermal expansion coefficient close to that of the siliconsubstrate 4 and contains few radioactive isotopes.

Furthermore, the supporting substrate 16 is preferably made of a highlytransparent material. If the supporting substrate 16 is transparent, itbecomes possible to detect the alignment marks M from the front-surfaceside of the back-illuminated type imaging device 100 by using anexposure apparatus having alignment mechanisms above and below theexposure apparatus, thereby forming the components on the back-surfaceside with reference to the alignment marks M. In this case, it is notnecessary to form the alignment marks M in the silicon substrate 4 butthe alignment marks M may be formed on the front surface of the siliconsubstrate 4.

A glass substrate can be exemplified as the highly transparent material.However, it is hard to be used since its thermal expansion coefficientdiffers from that of silicon and it contains many radioactive isotopes.Therefore, silicon is preferably used for the supporting substrate 16since the silicon's thermal expansion coefficient is close to that ofthe silicon substrate 4 and the silicon contains few radioactiveisotopes. However, since the silicon is not transparent, it cannot beused with the exposure apparatus having the alignment mechanisms aboveand below the exposure apparatus. Therefore, in this embodiment, thealignment marks M that are visible from the back-surface side areformed.

In a portion of the silicon substrate 4 where the imaging area 30 islocated, a large number of pixel portions are formed two-dimensionally.Each pixel portion includes a charge storage area 10 for storing thecharges, which are generated in the silicon substrate 4 in response tothe incidence light, and components (not shown) of a MOS circuit havingMOS transistors for reading out signals corresponding to the chargesstored in the charge storage area 10. Moreover, pixel separation areas11 for separating the pixel portions 10 from each other are formed on aninner side of the front surface of the silicon substrate 4.

A highly doped p-type impurity region 3 for blocking dark current isformed on an inner side of the back surface of the silicon substrate 4.In the pad forming areas of the silicon substrate 4, a plurality of thepillars 9 (only nine pillars are shown in the drawing for the sake ofsimplicity) extending from the back surface of the silicon substrate 4and reaching the front surface are formed below the large number of padportions 17, thereby electrically connecting the pad portions 17 to theplurality of pillars 9. In the mark forming area of the siliconsubstrate 4, the plurality of pillars 8 (only five pillars are shown inthe drawing for the sake of simplicity) extending from the back surfaceof the silicon substrate 4 and reaching the front surface are formed inpositions where the plurality of alignment marks M are to be formed. Thepillars 8 and 9 have the same size, when viewed in a plan view.Preferably, in view of easiness of fabrication, portions of the pillarswhere they have the maximum width have 2 μm or less in length (in thecase of a circular column, they have a diameter equal to or less than 2μm). The side walls of the pillars 8 and 9 are covered with insulatingfilms.

Also, the pillars 8 and 9 may have mutually different sizes.Furthermore, the respective pillars 9 connected to the pad portions 17may not have the same size, that is, may have different sizes. However;in any case, it is preferable that the size of the pillars 8 and 9 isset equal to or smaller than 2 μm. Also, the pillars 8 do notnecessarily extend from the back surface of the silicon substrate 4 andreach the front surface. The pillars 8 may be buried in the siliconsubstrate 4 so that the pillars 8 are detectable from both the front andback-surface sides of the silicon substrate 4. From the same reason, thepillars 8 are not necessarily made of the conductive material similar tothe pillars 9. However, it is to be noted that if the pillars 8 and 9are made formed of a columnar conductive material that extends from theback surface of the silicon substrate 4 and reach the front surface,they can be formed simultaneously with each other, which is advantageousfor the fabrication process.

The back-illuminated type imaging device 100 further includes: an oxidefilm 2 formed on the back surface of the silicon substrate 4 except thepad portions 17; a light shielding film 18 formed on the oxide film 2above pixel portions for black level detection; a light shielding film19 for preventing color mixing between pixels, formed on the oxide film2 above the pixel separation areas 11, which separate the pixelportions, excluding the pixel portions for black level detection; thecolor filters 20 formed on the oxide film 2 above the pixel portions;the microlenses 21 formed on the color filters 20; and a material film22 made of the materials, which are used to form the color filters 20and the microlenses 21. In the material film 22, openings are formed onthe pad portions 17, and the pad portions 17 are exposed through theopenings.

It is preferable that the pad portions 17, the light shielding film 18,and the light shielding film 19 are made of the same material. If theyare made of the same material, they can be formed at the same processstep, which advantageous for the fabrication process. As a material thathas light shielding property and allows the pad portions 17 to functionproperly, aluminum can be exemplified.

In the insulating film 14, formed are the wiring portions 12 such asvarious wrings connected to the components of the MOS circuit, whichoutputs signals corresponding to the charges stored in the chargestorage areas 10 of the silicon substrate 4, and connected to aperipheral circuit. In the example shown in the figure, the wiringportions 12 have a three-layer wiring structure. The wiring portions 12include contact wirings 13 that (i) connect various wirings connected tothe components of the MOS circuit and the peripheral circuit and (ii)the pillars 9 to each other. With this configuration, the wiringportions 12 and the pad portions 17 are electrically connected to eachother via the pillars 9.

Next, a method for fabricating the back-illuminated type imaging device100 having such configuration will be described.

FIGS. 3 to 8 are schematic sectional views showing the back-illuminatedtype imaging device 100 at respective fabrication process steps.

First, as shown in FIG. 3, an SOI substrate is prepared to include then-type silicon substrate 4 in which a p-type impurity layer 3 is dopedin an inner side from the back surface, an n-type silicon substrate 1,and an oxide film 2 formed between the back surface of the siliconsubstrate 4 and the silicon substrate 1.

Next, a SiO₂ film, for example, is formed on the front surface of thesilicon substrate 4 to form the insulating film 5, and a resist maskpattern is formed on the insulating film 5 by a photolithographicprocess. The resist mask pattern has openings formed thereon inpositions where the pillars 8 and 9 are to be formed. Subsequently,portions of the insulating film 5, the silicon substrate 4 and the oxidefilm 2 which are below the mask openings are selectively etched andremoved via the resist mask pattern to form through holes T having acircular column shape in a mark forming area 6 and a pad forming area 7(see FIG. 4). The resist mask pattern is designed so that the throughholes T have a diameter of 0.8 μm, for example.

Next, the side walls of the through holes T are oxidized to form aninsulating film on the side walls. A doped polysilicon film is formed asa conductive material layer on the insulating film 5. Subsequently, thedoped polysilicon film is etched back to bury the dope polysilicon inthe through holes T, and exposed surfaces of the doped polysilicon areoxidized to form the pillars 8 and 9 (see FIG. 5).

Next, the alignment marks M formed of the pillars 8 are detected fromthe front-surface side of the silicon substrate 4, and with reference tothe detected alignment marks M, the pixel portions including the chargestorage areas 10 and the pixel separation areas 11 are formed in theinner side of the silicon substrate 4 from the front surface of thesilicon substrate 4 by a conventional process. Subsequently, thecomponents of the MOS circuit, the peripheral circuit, and the wiringportions 12 are formed on the insulating film 5, and the insulating film14 is formed thereon to planarize the surface (see FIG. 6). In thiscase, the contact wirings 13 of the wiring portions 12 are formed byforming openings in portions of the insulating film 5 above the pillars9 and filling the openings with polysilicon.

Next, the supporting substrate 16 (made of silicon) is prepared to haveon a front surface thereof an adhesive film 15 that is made of SiO₂ andformed by the CVD process. The supporting substrate 16 is bonded to theinsulating film 14 via the adhesive film 15 by a direct bonding method(see FIG. 7). Subsequently, the silicon substrate 1 is etched andremoved to expose the oxide film 2 with the supporting substrate 16being used as a base layer (see FIG. 8).

Next, portions of the oxide film 2 on the pad forming areas 7 are etchedand removed to expose the pillars 9. Thereafter, an aluminum film, forexample, is formed as a conductive material layer on the oxide film 2.Subsequently, a resist mask pattern is formed on the aluminum film sothat the resist mask pattern has openings in positions outside (i) thepad forming areas 7, (ii) a portion above the pixel portions for blacklevel detection and (iii) a portion above the pixel separation areas 11between the pixel portions. Then, the aluminum film is etched via theresist mask pattern to form pad portions 17 and the light shieldingfilms 18 and 19.

Next, in order to secure the electrical connection between the dopedpolysilicon constituting the pillars 9 and the aluminum filmconstituting the pad portions 17, a heating treatment (sintering) isperformed in a hydrogen-containing atmosphere at a temperature around400° C. Subsequently, the color filters 20 and the microlenses 21 areformed. Finally, openings are formed at portions of the material film22, which is formed when the color filters 20 and the microlenses 21 areformed, above the pad portions 17 to expose the pad portions 17.Thereby, the back-illuminated type imaging device 100 shown in FIG. 1 isobtained.

As described above, according to the back-illuminated type imagingdevice 100, the device has the structure that the pad portions 17 formedon the back-surface side of the semiconductor substrate 4 and the wiringportions 12 on the front-surface side are electrically connected to eachother via the plurality of pillars 9. Therefore, it is possible toperform the electrical connection between the pad portions 17 and thewiring portions 12 while suppressing the resistance between the padportions 17 and the wiring portions 12 to a low level even if the sizeof the pillars 9 is not increased. Since the size of the pillars 9 canbe decreased (for example, to 2 μm or less), if the above-describedfabrication method is employed, it is possible to decrease the processload of filling the through hole T with the conductive material, whichreduces the fabrication cost.

Moreover, according to the back-illuminated type imaging device 100, thealignment marks M are formed of the pattern of the plurality of smallerpillars 8. Therefore, it is possible to decrease the process load offorming the alignment marks M, which decreases the fabrication cost. Inaddition, since the pillars 8 are made of the same material and have thesame length as the pillars 9, it is possible to form the pillars 8simultaneously with the pillar 9 as described in the fabrication method,which reduces the fabrication cost.

In addition, according to the back-illuminated type imaging device 100,the semiconductor substrate 4 and the supporting substrate 16 are bondedto each other via an inorganic material. Therefore, a sintering processfor securing the electrical connection between the pad portions 17 andthe pillars 9 does not weaken the adhesion force between thesemiconductor substrate 4 and the supporting substrate 16 or bend thesupporting substrate 16. That is, it is possible to form the colorfilter 20 or the microlens 21 in a state where the supporting substrate16 is completely parallel to the semiconductor substrate 4. Therefore,it is possible to form the color filter 20 or the microlens 21 asoriginally designed, which improves a yield ratio and reduces thefabrication cost.

In addition, according to the back-illuminated type imaging device 100,the plurality of pillars 9 connected to the pad portions 17 are arrangedtwo-dimensionally. Therefore, it is possible to provide advantages thatthe electrical resistance is decreased and that bonding properties tothe pad electrode are improved.

In addition, according to the back-illuminated type imaging device 100,the supporting substrate 16 and the semiconductor substrate 4 are madeof the same material. Therefore, it is possible to provide advantagesthat post-generated white defects can be prevented and that damages tothe semiconductor substrate 4, which may be caused during a sinteringprocess, can be reduced.

In addition, according to the back-illuminated type imaging device 100,the pad portions 17 and the light shielding films 18 and 19 are made ofthe same material. Therefore, it is possible to form the pad portionssimultaneously with the light shielding films as described in the abovefabrication method, which reduces the fabrication cost.

In addition, although the back-illuminated type imaging device 100 hasbeen described as being a MOS type imaging device, the imaging devicemay be a CCD type imaging device. Also, marks for inspectingmisalignment of the components on the back-surface side may be formed inthe semiconductor substrate 4. These marks may be formed of a pluralityof pillars.

1. A back-illuminated type imaging device comprising: a semiconductorsubstrate including a front surface and a back surface, wherein light isilluminated from the back-surface side of the semiconductor substrate,and charges that are generated in response to the light are read outfrom the front-surface side of the semiconductor substrate to performimaging; a plurality of pad portions formed on the back surface of thesemiconductor substrate; a plurality of wiring portions formed on thefront surface of the semiconductor substrate; and a plurality of contactwiring portions that are formed in the semiconductor substrate andelectrically connect the wiring portions formed on the front surface ofthe semiconductor substrate and the pad portions.
 2. Theback-illuminated type imaging device according to claim 1, wherein eachcontact wiring portion is configured by a pillar made of a conductivematerial, the pillar extending from the front surface of thesemiconductor substrate in a direction perpendicular to the frontsurface of the semiconductor substrate, the pillar reaching the backsurface of the semiconductor substrate, and the pad portions areconnected to one ends of the pillars, and the wiring portions areconnected to the other ends of the pillars.
 3. The back-illuminated typeimaging device according to claim 2, wherein the plurality of pillarsare arranged two-dimensionally, when viewed in plan view.
 4. Theback-illuminated type imaging device according to claim 2, whereinportions of the plurality of pillars at which the pillars have a maximumwidth have 2 μm or less in length.
 5. The back-illuminated type imagingdevice according to claim 1, further comprising: an alignment mark thatis formed in the semiconductor substrate and is used to align componentson the front-surface side of the semiconductor substrate with componentson the back-surface side, wherein the alignment mark is formed by apattern of a plurality of pillars that are made of a conductive materialand extend in a direction perpendicular to the front surface of thesemiconductor substrate.
 6. The back-illuminated type imaging deviceaccording to claim 5, wherein the plurality of pillars, which form thealignment mark, are arranged two-dimensionally.
 7. The back-illuminatedtype imaging device according to claim 5, wherein the pattern of theplurality of pillars, which form the alignment mark, is symmetric. 8.The back-illuminated type imaging device according to claim 5, whereinportions of the plurality of pillars at which the pillars have a maximumwidth have 2 μm or less in length.
 9. The back-illuminated type imagingdevice according to claim 5, wherein each of the pillars, which form thealignment mark, extends from the front surface of the semiconductorsubstrate and reaches the back surface of the semiconductor substrate.10. The back-illuminated type imaging device according to claim 1,further comprising: a supporting substrate bonded to the front surfaceof the semiconductor substrate via an inorganic adhesive layer.
 11. Theback-illuminated type imaging device according to claim 10, wherein thesupporting substrate is made of the same material as the semiconductorsubstrate.
 12. The back-illuminated type imaging device according toclaim 1, further comprising: a film that is made of the same material asthe pad portions and is formed above the back surface of thesemiconductor substrate, wherein the film includes at least one of (i) alight shielding film for shielding a part of pixel portions formed inthe semiconductor substrate from light and (ii) a light shielding filmfor shielding a boundary between the pixel portions from light.
 13. Amethod for fabricating a back-illuminated type imaging device in whichlight is illuminated from a back-surface side of a semiconductorsubstrate, and charges that are generated in response to the light areread out from a front-surface side of the semiconductor substrate toperform imaging, the method comprising: forming a plurality of firstthrough holes in a plane area, on which pad portions to be formed on theback surface of the semiconductor substrate is to be formed, from thefront-surface side of the semiconductor substrate so that the throughholes extends from the front surface of the semiconductor substrate andreaches the back surface; filling the plurality of first through holeswith a conductive material to form a plurality of first pillars made ofthe conductive material; forming wiring portions on the front surface ofthe semiconductor substrate so that the wiring portions are connected tothe plurality of first pillars; and after the wiring portions areformed, forming the pad portions on the back surface of thesemiconductor substrate so that the pad portions are connected to theplurality of first pillars.
 14. The method for fabricating theback-illuminated type imaging device according to claim 13, wherein inthe forming of the first through holes, the plurality of first throughholes are formed so as to be arranged two-dimensionally, when viewed inplan view.
 15. The method for fabricating the back-illuminated typeimaging device according to claim 13, wherein in the forming of thefirst through holes, the plurality of first through holes are formed sothat portions of the first through holes at which the through holes havea maximum width have 2 μn or less in length.
 16. The method forfabricating the back-illuminated type imaging device according to claim13, wherein in the forming of the first through holes, a plurality ofsecond through holes are formed in a plane area, in which a alignmentmark that is used to align components on the front-surface side of thesemiconductor substrate with components on the back-surface side are tobe formed, so that the second through holes extends from the frontsurface of the semiconductor substrate and reaches the back surface, inthe filling of the first through holes, the plurality of second throughholes are filled with the conductive material to form a plurality ofsecond pillars made of the conductive material, and the alignment markis formed by the pattern of the plurality of second pillars.
 17. Themethod for fabricating the back-illuminated type imaging deviceaccording to claim 16, wherein in the forming of the first throughholes, the plurality of second through holes are formed so as to bearranged two-dimensionally, when viewed in plan view.
 18. The method forfabricating the back-illuminated type imaging device according to claim17, wherein in the forming of the first through holes, a pattern of theplurality of second through hole is formed so as to be symmetric. 19.The method for fabricating the back-illuminated type imaging deviceaccording to claim 16, wherein in the forming of the first throughholes, the plurality of second through holes are formed so that portionsof the second through holes at which the through holes have a maximumwidth have 2 μm or less in length.
 20. The method for fabricating theback-illuminated type imaging device according to claim 13, furthercomprising forming pixel portions in the semiconductor substrate, thepixel portions having charge storage areas for storing therein thecharges, wherein the forming of the pad portions includes forming a filmof a conductive material having a light shielding property, on the backsurface, and removing the conductive material at portions outside atleast one of a portion above a plane area in which the pad portions areformed, a portion above a part of the pixel portions, and a portionabove a boundary between the pixel portions.
 21. The method forfabricating the back-illuminated type imaging device according to claim20, further comprising after the pad portions are formed, forming colorfilters above the pixel portions, so as to correspond to the pixelportions; and after the pad portions are formed and before the colorfilters are formed, heating the semiconductor substrate.
 22. The methodfor fabricating the back-illuminated type imaging device according toclaim 13, wherein the semiconductor substrate is a first semiconductorsubstrate of a SOT substrate, the SOI substrate comprising the firstsemiconductor substrate, a second semiconductor substrate, and an oxidefilm sandwiched between the first and second semiconductor substrates,the method further comprising: after the wiring portions are formed,bonding a supporting substrate on the front-surface side of the firstsemiconductor substrate via an inorganic adhesive film.
 23. The methodfor fabricating the back-illuminated type imaging device according toclaim 22, wherein the supporting substrate is made of the same materialas the first semiconductor substrate.